US12354778B2 - Grain-oriented electrical steel sheet and method for producing same - Google Patents

Grain-oriented electrical steel sheet and method for producing same Download PDF

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US12354778B2
US12354778B2 US17/768,570 US202017768570A US12354778B2 US 12354778 B2 US12354778 B2 US 12354778B2 US 202017768570 A US202017768570 A US 202017768570A US 12354778 B2 US12354778 B2 US 12354778B2
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mass
steel sheet
film
annealing
grain
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US20240105369A1 (en
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Makoto Watanabe
Masahiro SUEMUNE
Takeshi Imamura
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JFE Steel Corp
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JFE Steel Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/147—Alloys characterised by their composition
    • H01F1/14766—Fe-Si based alloys
    • H01F1/14775—Fe-Si based alloys in the form of sheets
    • H01F1/14783—Fe-Si based alloys in the form of sheets with insulating coating
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/147—Alloys characterised by their composition
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/147—Alloys characterised by their composition
    • H01F1/14708—Fe-Ni based alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/16—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys in the form of sheets
    • H01F1/18—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys in the form of sheets with insulating coating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C2202/00—Physical properties
    • C22C2202/02—Magnetic
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • This invention relates to a grain-oriented electrical steel sheet and a method for producing the same, and more specifically, to a grain-oriented electrical steel sheet having excellent magnetic properties and film properties and being suitable for a magnetic domain subdividing treatment and a method for producing the same.
  • Grain-oriented electrical steel sheets are mainly used as an iron core material for a transformer and thus strongly demanded to have excellent magnetic properties, particularly a low iron loss.
  • Such a grain-oriented electrical steel sheet has been produced by subjecting a cold-rolled Si-containing steel sheet to decarburization annealing combined with primary recrystallization annealing, applying an annealing separator composed mainly of MgO thereto, and causing secondary recrystallization in finish annealing to highly align crystal grains into ⁇ 110 ⁇ 001> orientation (so-called Goss orientation).
  • Patent Literature 4 proposes a method in which, in flattening annealing, each condition of the soaking temperature in annealing, cooling rate from the soaking temperature, and plastic elongation amount of the steel sheet is adjusted to control decreasing amount in the tension of the forsterite film before and after the flattening annealing to not more than 60% and further adding compounds of Sn, Sb, Mo and W to an annealing separator to include these metals in the iron matrix.
  • Patent Literature 1 The method disclosed in Patent Literature 1 is based on the knowledge that the irradiation of plasma flame or a laser beam, even when Se is concentrated on the steel sheet surface to improve the film properties, tends to cause film breakage while the electron beam irradiation applies little heat to the forsterite film and is suitable to the magnetic domain subdividing treatment. Accordingly, the method cannot be applied to a method other than the electron beam irradiation method, for example, a magnetic domain subdividing treatment through laser beams or plasma flame. In addition to concentrating Se, concentrating S, Al and the like are necessary to be taken into consideration, but it is difficult to control all the elements into given ranges.
  • Patent Literature 2 is limited to the magnetic domain subdividing treatment conducted by irradiating electron beams, and the effect to the magnetic domain subdividing treatment conducted by irradiating plasma flame or laser beams is unclear.
  • the method also has a problem that, even in the case of irradiating the electron beam, the stripping of the film cannot be completely prevented when irradiation energy is increased to enhance the effect of improving the iron loss properties.
  • the method has another problem that, as heating and cooling are necessary for a vacuum zone before and after the electron beam irradiation, the equipment costs and the running costs are increased.
  • the method of preventing the stripping of the film by the magnetic domain subdividing treatment is not still sufficient in terms of not only the practical use but also the effect.
  • it tends to increase the irradiation energy in the magnetic domain subdividing treatment to further enhance the effect of improving the iron loss property, and hence the conventional method of preventing the stripping of the film has failed to produce sufficient effects.
  • aspects of the invention are made in consideration of the above problems of the prior arts, and an object thereof is to provide a grain-oriented electrical steel sheet capable of securing film adhesiveness even when magnetic domain subdividing treatment is conducted at a high energy density and propose a method for advantageously producing the same.
  • the inventors have studied focusing on what properties of the film and iron matrix are involved in the stripping of the film due to the magnetic domain subdivision. As a result, they have found a new knowledge that both the pickling weight loss caused by pickling the forsterite undercoat film with HCl and the total concentration of Sn, Sb, Mo, and W present in a boundary face between the undercoat film and iron matrix (steel sheet surface) have a large influence on the stripping properties of the film due to the magnetic domain subdividing treatment, and developed the invention.
  • aspects of the invention include
  • aspects of the present invention prevent the stripping of the film even when magnetic domain subdividing treatment is performed at an energy density higher than that in the prior arts, which can not only improve the corrosion resistance and insulation properties of a product sheet but also provide a better effect of reducing iron loss.
  • FIG. 1 is photographs showing surface reflection electron images of steel sheet surfaces having different pickling weight losses of undercoat films after pickling, and secondary electron images of film sections.
  • FIGS. 2 ( a ) to 2 ( d ) are graphs showing influences of finish annealing conditions upon pickling weight loss of a steel sheet having an undercoat film and upon the Sn concentration in a boundary face between the film and iron matrix.
  • FIGS. 3 ( a ) and 3 ( b ) are graphs showing an influence of a furnace pressure of an atmospheric gas at heating in a high-temperature zone of finish annealing upon a relation of the current density of electron beam irradiation, stripping properties of the film, and iron loss.
  • FIGS. 4 ( a ) and 4 ( b ) are different graphs showing an influence of a furnace pressure of an atmospheric gas at heating in a high-temperature zone of finish annealing upon a relation of the current density of electron beam irradiation, stripping properties of the film, and iron loss.
  • the obtained cold-rolled sheet is then subjected to decarburization annealing combined with primary recrystallization annealing at 830° C. for 100 seconds under a wet atmosphere of 50 vol % H 2 -50 vol % N 2 having a dew point of 57° C.
  • a slurried annealing separator composed mainly of MgO and added with SnO 2 by 3 mass % as converted to Sn as an additive is applied to the steel sheet surface and dried.
  • the steel sheet is then subjected to finish annealing of conducting retention treatment of developing secondary recrystallization and purification treatment of holding a temperature of 1150° C. in H 2 atmosphere for 20 hours to produce a grain-oriented electrical steel sheet having a forsterite undercoat film.
  • the H 2 concentration in the atmospheric gas is varied within the range of 0 to 10 vol %
  • the retention treatment temperature is varied within the range from 750° C. to 1050° C.
  • the retention treatment time is varied within a range of 5 to 120 hours.
  • a dry gas of 98 vol % N 2 +2 vol % H 2 having a dew point of ⁇ 5° C. is introduced into the furnace as the atmospheric gas while the furnace pressure is varied within the range of 1.5 to 6 mmH 2 O.
  • the forsterite film contains at least 50 mass % forsterite.
  • the steel sheet after the finish annealing is pickled by varying the immersion time in the 5% HCl aqueous solution at 60° C.
  • an oxygen coating weight g/m 2
  • the forsterite film is assumed to be removed up to the boundary face between the forsterite film and iron matrix, and the Sn concentration on the steel sheet surface (iron matrix) is quantified by GDS and is defined as the Sn concentration in the “boundary face between the film and iron matrix”.
  • the oxygen coating weight means an oxygen weight per unit area (both sides) on an assumption that the analyzed oxygen content in the total thickness of the steel sheet with the forsterite film is present on the steel sheet surface.
  • An insulation film is applied to the surface of the steel sheet subjected to the finish annealing, and then the steel sheet is dried and subjected to flattening annealing at 800° C. for 60 seconds combined with baking and flattening treatment.
  • the steel sheet after the annealing is subjected to a magnetic domain subdividing treatment by irradiating laser beams on the steel sheet surface at an irradiation energy of 1.5 mJ/mm 2 to produce a product sheet.
  • the surface of the steel sheet after the magnetic domain subdividing treatment is observed by an optical microscope at 10 magnifications to examine the occurrence of the film stripping, which is compared with the pickling weight loss of the steel sheet after the finish annealing.
  • the result reveals, as shown in Table 1, that no film stripping is caused when the pickling weight loss is not more than 1.8 g/m 2 .
  • the photographs at the upper part of FIG. 1 are reflection electron images when the steel sheet surfaces after the finish annealing having two different pickling weight losses of 1.5 g/m 2 and 1.9 g/m 2 are observed by an SEM (scanning type electron microscope), which show that the film stripping is partially caused (white portions in the photograph) in the steel sheet having a high pickling weight loss as 1.9 g/m 2 , while the whole film thickness is uniformly thin in the steel sheet having the low pickling weight loss as 1.5 g/m 2 .
  • the photographs at the lower part of FIG. 1 are secondary electron images of the cross-section of the film of the above two steel sheets observed by the SEM.
  • the figure shows that the boundary face between the film and iron matrix is corroded by the pickling to form gaps in the steel sheet having the high pickling weight loss, while such gaps are not formed on the boundary face between the film and iron matrix of the steel sheet having low pickling weight loss after the pickling. Accordingly, it is revealed that the pickling weight loss correlates with the adhesiveness of the undercoat film, that is, the pickling weight loss is an indication that represents the adhesiveness of the undercoat film, and the adhesiveness of the film of the steel sheet after magnetic domain subdividing treatment can be predicted by measuring the pickling weight loss of the steel sheet after the finish annealing.
  • the Sn concentration on the boundary face between the film and iron matrix of the steel sheet after the finish annealing i.e., the Sn concentration of the steel sheet (iron matrix) surface (boundary face between the film and iron matrix) when the forsterite film is completely removed by pickling with HCl falls within the range of 0.01 to 0.15 mass %.
  • the inventors consider the reason thereof as follows. Sn present on the boundary face between the film and iron matrix, i.e., the steel sheet (iron matrix) surface is considered to have an effect of increasing a high-temperature strength of the steel sheet to thus reduce the deformation quantity of the steel sheet (iron matrix) by heat energy received in the magnetic domain subdividing treatment.
  • the above effect cannot be sufficiently obtained with the Sn concentration being less than 0.01 mass %, while the properties of the film itself are deteriorated and the good adhesiveness of the film is not obtained with the Sn concentration being more than 0.15 mass %.
  • SnO 2 added to the annealing separator remains in the forsterite film or is decomposed into Sn, which is dispersed toward the steel sheet side to remain on the boundary face between the film and iron matrix or penetrate into the steel sheet.
  • Sn concentration remaining on the boundary face between the film and iron matrix that has a large influence on the film stripping by magnetic domain subdividing treatment.
  • SnO 2 amount in the annealing separator is set constant (3 mass %), the Sn concentration on the boundary face between the film and iron matrix varies in this experiment. This means that it is necessary to establish a technique of controlling the concentration of Sn or the like present on the boundary face between the film and iron matrix to a proper range to prevent the film stripping by the magnetic domain subdividing treatment.
  • the inventors have studied a method of controlling the pickling weight loss and Sn concentration on the boundary face between the film and iron matrix to proper ranges based on the above experiment results.
  • FIG. 2 shows the experiment result and reveals the following.
  • the retention treatment time has a tendency similar to the retention treatment temperature, that is, it does not largely affect the Sn concentration on the boundary face between the film and iron matrix but largely affects the pickling weight loss.
  • the pickling weight loss increases, and there is a proper range to minimize the pickling weight loss ( FIG. 2 ( b ) ).
  • the H 2 concentration in the atmospheric gas during the retention treatment affects both the pickling weight loss and the Sn concentration on the boundary face between the film and iron matrix. As the H 2 concentration is higher, the pickling weight loss tends to more increase while the Sn concentration on the boundary face between the film and iron matrix more decreases ( FIG. 2 ( c ) ).
  • the pressure inside the furnace (furnace pressure) of Hz-containing atmospheric gas at the heating in the high-temperature zone of from 1050° C. to the purification treatment temperature also affects both the pickling weight loss and the Sn concentration on the boundary face between the film and iron matrix. As the furnace pressure is higher, the pickling weight loss more decreases while the Sn concentration on the boundary face between film and iron matrix more increases ( FIG. 2 ( d ) ).
  • the inventors have the following thoughts on the result.
  • the forsterite film is formed slowly during the retention treatment, resulting in finer and denser forsterite grain size and improved grain boundary strength. As a result, the progression of the corrosion by the pickling is delayed to decrease the pickling weight loss.
  • the retention treatment temperature is too low, however, the forsterite film is hardly formed in the retention treatment temperature range and rapidly reacts at the subsequent heating process, causing a coarse structure having large gaps.
  • the retention treatment temperature is too high, the reaction rate of the film formation is increased, to cause coarse forsterite grain size and weaken grain boundary strength, resulting in an increase in the pickling weight loss.
  • Sn is concentrated on the boundary face between the film and iron matrix. Adding Sn to the raw steel material can increase the Sn concentration on the boundary face between the film and iron matrix, but causing problems that the rolling properties are deteriorated and the surface defect is caused. Even if Sn of the target concentration of 0.01 to 0.15 mass % is added to the raw steel material, it is absorbed in the forsterite film during the finish annealing, and hence the Sn concentration on the boundary face between the film and iron matrix after the finish annealing is reduced to less than 0.01 mass %.
  • C is in the range of 0.02 to 0.08 mass %, preferably in the range of 0.025 to 0.075 mass %.
  • Si is an element necessary for increasing the specific resistance of steel, thus reducing iron loss. This effect is not sufficient with the content of less than 2.5 mass %, while when it exceeds 4.5 mass %, the workability of steel is deteriorated to make the production by rolling difficult. Therefore, Si falls within the range of 2.5 to 4.5 mass %, preferably 2.8 to 4.0 mass %.
  • Mn is an element necessary for improving the hot workability of steel. This effect is insufficient with the C content of less than 0.03 mass %, while when it exceeds 0.30 mass %, the magnetic flux density of a product sheet lowers. Therefore, Mn falls within the range of 0.03 to 0.30 mass %, preferably 0.04 to 0.20 mass %.
  • the inhibitor is preferable to contain Al: 0.010 to 0.040 mass % and N: 0.003 to 0.012 mass %.
  • the inhibitor is preferable to include the above-described Mn content and one or two selected from S: 0.0040 to 0.030 mass % and Se: 0.0030 to 0.030 mass %.
  • S 0.0040 to 0.030 mass %
  • Se 0.0030 to 0.030 mass %.
  • the remainder other than the above ingredients is Fe and inevitable impurities.
  • Ni 0.01 to 1.50 mass %
  • Cr 0.01 to 0.50 mass %
  • Cu 0.01 to 0.50 mass %
  • P 0.005 to
  • the steel having the aforementioned component composition is melted by a usual refining process.
  • the steel may be formed into a raw steel material (slab) by the conventionally well-known ingot making-blooming method or continuous casting method or may be formed into a thin cast strip having a thickness of not more than 100 mm by a direct casting method.
  • the slab is heated according to the usual manner. For example, it is heated to a temperature not lower than about 1350° C. when containing the inhibitor ingredient or to a temperature not higher than 1300° C. when containing no inhibitor ingredient and hot-rolled under the conventionally well-known conditions. When containing no inhibitor ingredient, the slab may be hot-rolled immediately after the casting without heating. In the case of the thin cast strip, it may be hot-rolled or may proceed to subsequent steps without the hot rolling.
  • the hot-rolled sheet obtained by the hot rolling is subjected to hot-band annealing as necessary.
  • the hot-band annealing is preferably conducted at an annealing temperature of 800 to 1150° C. in order to obtain good magnetic properties.
  • the annealing temperature is lower than 800° C., a band structure formed by the hot rolling remains, thus failing to obtain a primary recrystallization texture of regulated grains, and thus the development of secondary recrystallization is blocked.
  • the annealing temperature exceeds 1150° C., the grain size after the hot-band annealing is too coarsened to obtain the primary recrystallization texture of regulated grains as well.
  • the decarburization annealing decreases C in steel to not more than 0.0050 mass % with which no magnetic aging is caused.
  • the steel sheet subjected to the finish annealing satisfying the above conditions has the pickling weight loss of the forsterite film of not more than 1.8 g/m 2 , which hardly causes the film stripping by the magnetic domain subdivision. Also, the steel sheet subjected to the finish annealing satisfying the above conditions has a total concentration (converted to metal) of Sn, Sb, Mo, and W on the boundary face between the film and iron matrix of 0.01 to 0.15 mass %, which hardly causes the film stripping even when subjected to high-density heat energy by the magnetic domain subdivision.
  • the pickling weight loss of the grain-oriented electrical steel sheet (product sheet) that has an insulation film formed on the forsterite undercoat film it is necessary to conduct the measurement after removing the insulation film with hot alkali.
  • a steel slab having a component composition comprising C: 0.070 mass %, Si: 3.43 mass %, Mn: 0.08 mass %, Al: 0.005 mass %, N: 0.004 mass %, S: 0.002 mass %, Sb: 0.02 mass % and the remainder being Fe and inevitable impurities is produced by a continuous casting method, heated to 1250° C., and hot-rolled to form a hot-rolled sheet with a sheet thickness of 2.4 mm. The hot-rolled sheet is then subjected to a primary cold rolling to an intermediate sheet thickness of 1.8 mm, an intermediate annealing at 1100° C.
  • decarburization annealing is conducted at 840° C. for 100 seconds under a wet atmosphere of 50 vol % H 2 -50 vol % N 2 having a dew point of 55° C.
  • a slurried annealing separator composed mainly of MgO and containing various amounts of compounds of Sn, Sb, Mo, and W as an additive as shown in Table 2 is applied to the steel sheet surface after the decarburization annealing and is dried.
  • the steel sheet is then subjected to finish annealing comprised of retention treatment of holding the steel sheet at 920° C. for 80 hours under an Ar atmosphere and purification treatment conducted at 1200° C. for 10 hours in an H 2 atmosphere.
  • a dry gas containing 20 vol % H 2 and having a dew point of ⁇ 20° C. as an atmospheric gas is introduced such that the pressure inside the furnace (furnace pressure) reaches 6 mmH 2 O at the heating from 1050° C. to the purification treatment temperature (at the heating in the high-temperature zone).
  • the steel sheet having a forsterite film after the finish annealing is pickled by immersing in an aqueous solution of 5% HCl at 60° C. for 60 seconds as in Experiment 1 to measure a pickling weight loss (g/m 2 ) of the undercoat film thereof, while after conducting pickling with hydrochloric acid to reduce the coating weight converted to oxygen to 5 to 10% of that before the pickling, the total concentration of Sn, Sb, Mo and W on the boundary face between the film and iron matrix are measured under the condition of 20 kV and 2 mA in the air with a fluorescent X-ray measuring device and qualified by a calibration curve previously prepared.
  • the steel sheet after the finish annealing is coated with an insulation film, subjected to flattening annealing combined with baking and a flattening treatment, and magnetic domain subdividing treatment by irradiating electron beams to the steel sheet surface at a current density of 80 mA/mm 2 to produce a product sheet, on which whether film stripping is caused is examined by means of an optical microscope (10 magnifications).
  • Table 2 shows the measurement result. It reveals that applying the aspects of present invention prevents the film stripping by magnetic domain subdividing treatment.
  • a steel slab having a component composition comprising C: 0.06 mass %, Si: 3.25 mass %, Mn: 0.07 mass %, Al: 0.015 mass %, N: 0.006 mass %, S: 0.002 mass %, Cu: 0.08 mass % and the remainder being Fe and inevitable impurities is produced by a continuous casting method, heated to 1380° C., hot-rolled to form a hot-rolled sheet having a sheet thickness of 2.4 mm, and subjected to hot-band annealing at 1000° C. for 50 seconds, a primary cold rolling to an intermediate sheet thickness of 1.8 mm, an intermediate annealing at 1060° C.
  • a slurried annealing separator composed mainly of MgO and containing WO 3 by 1 mass % as converted to W is applied to the surface of the steel sheet after the decarburization annealing and is dried. Then, the steel sheet is subjected to finish annealing comprised of retention treatment of holding the steel sheet at 920° C. for 50 hours under an Ar atmosphere and purification treatment conducted at 1200° C. for 10 hours under an H 2 atmosphere. At the heating in the high-temperature zone of from 1050° C.
  • the pressure inside the furnace (furnace pressure) is controlled to 6 mmH 2 O, while the H 2 concentration and the dew point of an atmospheric gas introduced into the furnace are varied within ranges of the H 2 concentration: 0 to 80 vol % and the dew point: ⁇ 50 to 20° C. as shown in Table 3.
  • Steel slabs having various component compositions as shown in Table 4 and the remainder being Fe and inevitable impurities are produced by a continuous casting method.
  • the slabs containing inhibitor-forming ingredients (Nos. 1 to 3 in Table 4) and the slabs containing inhibitor-forming ingredients (Nos. 4 to 24 in Table 4) are heated to 1200° C. and 1380° C., respectively, each hot-rolled to form a hot-rolled sheet having a sheet thickness of 2.0 mm, subjected to hot-band annealing at 1030° C. for 10 seconds, cold-rolled to form a cold-rolled sheet having a final sheet thickness of 0.23 mm, and subjected to decarburization annealing combined with a primary recrystallization annealing.
  • the decarburization annealing is conducted at 840° C. for 100 seconds under a wet atmosphere of 50 vol % H 2 -50 vol % N 2 having a dew point of 61° C.
  • a slurried annealing separator composed mainly of MgO and containing MoO 3 by 2 mass % as converted to Mo is applied to the steel sheet surface after the decarburization annealing and is dried. Then, the steel sheet is subjected to finish annealing comprised of retention treatment of holding the steel sheet at 920° C. for 50 hours under an Ar atmosphere and purification treatment conducted at 1200° C. for 10 hours under an H 2 atmosphere. At the heating in the high-temperature zone from 1050° C. to the purification treatment temperature, a dry gas containing 75 vol % H 2 and having a dew point of ⁇ 20° C. as an atmospheric gas is introduced into the furnace such that the pressure inside the furnace (furnace pressure) reaches 6 mmH 2 O.
  • the steel sheet after the finish annealing is coated in an insulation coating, subjected to flattening annealing combined with baking and flattening treatment, and subjected to magnetic domain subdividing treatment by irradiating electron beams to the steel sheet surface at a current density of 80 mA/mm 2 to produce a product sheet, on which whether film stripping is caused is examined as in Example 1.
  • Table 4 shows measurement results of the pickling weight loss, total concentration of Sn, Sb, and Mo on the boundary face between the film and iron matrix, and whether or not the film stripping is caused.
  • the finish annealing conducted under conditions in conformity with the present invention prevents the film stripping by magnetic domain subdividing treatment.
  • a steel slab having a component composition comprising C: 0.06 mass %, Si: 3.4 mass %, Mn: 0.08 mass %, Al: 0.025 mass %, N: 0.008 mass %, Se: 0.02 mass %, Sb: 0.05 mass % and the remainder being Fe and inevitable impurities is produced by a continuous casting method, heated to 1420° C., and hot-rolled to form a hot-rolled sheet having a sheet thickness of 2.5 mm. The hot-rolled sheet is then subjected to hot-band annealing at 1000° C. for 50 seconds, primary cold rolling to an intermediate sheet thickness of 1.5 mm, intermediate annealing at 1100° C.
  • decarburization annealing is conducted at 840° C. for 100 seconds under a wet atmosphere of 50 vol % H 2 -50 vol % N 2 having a dew point of 58° C.
  • a slurried annealing separator composed mainly of MgO and containing MoO 3 by 4 mass % as converted to Mo is applied to the steel sheet surface after the decarburization annealing and is dried. Then, the steel sheet is subjected to finish annealing comprised of retention treatment of holding the steel sheet at 920° C. for 40 hours under a dry N 2 atmosphere and purification treatment conducted at 1200° C. for 10 hours under an H 2 atmosphere.
  • a dry gas containing 1 vol % H 2 and having a dew point of ⁇ 5° C. as an atmospheric gas is introduced into the furnace, such that the pressure inside the furnace (furnace pressure) reaches two conditions, 1.5 mmH 2 O and 6 mmH 2 O.
  • Measurement is performed on the pickling weight loss (g/m 2 ) and on the total concentration of Sb and Mo on the boundary face between the film and iron matrix of the steel sheet having a forsterite film after the finish annealing, as in Example 1.
  • the measurement results are such that the pickling weight loss is 1.92 g/m 2 and 1.12 g/m 2 in the case of the furnace pressure of 1.5 mmH 2 O and 6 mmH 2 O, respectively, and the total concentration of Sb and Mo on the boundary face between the film and iron matrix is 0.008 mass % and 0.071 mass % in the case of the furnace pressure of 1.5 mmH 2 O and 6 mmH 2 O, respectively.
  • the steel sheet after the finish annealing is coated in an insulation coating, subjected to flattening annealing combined with baking and flattening treatment, and subjected to magnetic domain subdividing treatment by irradiating electron beams to the steel sheet surface at a current density varying within the range of 10 to 120 mA/mm 2 to produce a product sheet, on which whether film stripping is caused is examined as in Example 1 and of which iron loss W 17/50 is measured according to a measurement method of alternating-current magnetization properties of JIS C2550.
  • FIG. 3 shows the measurement result.
  • the finish annealing conducted under conditions in conformity with the present invention maximizes the effect of reducing the iron loss by magnetic domain subdividing treatment without causing film stripping even when electron beams are irradiated at a high energy density.
  • FIG. 4 shows the result of the above measurement.
  • the finish annealing conducted under conditions in conformity with the present invention maximizes the effect of reducing the iron loss by magnetic domain subdividing treatment without causing film stripping even when electron beams are irradiated at a high energy density.

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